Novel sub-ceiling temperature rapid depolymerization-repolymerization reactions of cyanoacrylate...

Macronwl. Rapid Cornnlun. 17,217-227 (19961 217

Novel sub-ceiling temperature rapid depolymerization- repolymerization reactions of cyanoacrylate polymers

Bernard Rvan *, Gerard McCann

Chemical and Polymer Science Department. Research Development and Engineering Group, Loctite (Ireland) Ltd.. Tallaght Business Park, Whitestown, Dublin 24, Ireland

(Received: December 1 I , 1995)

SUMMARY Dilute solutions of unimodal high-molecular-weight poly(buty1 cyanoacrylate) (“par-

ent” polymer) in tetrahydrofuran (THF) at 2 I “C underwent rapid depolymerization accompanied by simultaneous repolymerization of the unzipped monomer to yield lower- molecular-weight “daughter” polymer following addition of tetrabutylammonium hydroxide (TBAOH), a slow initiator of butyl cyanoacrylate (BCA) monomer. The rate-determining step for the reaction was tentatively identified as abstraction of the terminal chain proton. Similarly, addition of TBAOH to a THF solution of BCA monomer resulted in initial formation of high-molecular-weight (MW) polymer that subsequently rapidly depolymerized with simultaneous formation of TBAOH initiated, low-MW daughter polymer. The depolymerization-repolymerization reaction is the relaxation process that occurs following perturbation of the monomedhigh-MW parent polymer equilibrium by low-MW daughter polymer and proceeds at least 100°C below the ceiling temperature of the parent polymer.

Introduction

Alkyl cyanoacrylates are among the most reactive monomers known in anionic polymerization; propagation rate constants for both ethyl cyanoacrylate and butyl cyanoacrylate (BCA) of between 3 x los and 6 x lo5 L.mol-’.s-l at 20°C in tetrahydrofuran (THF) have been measured) ’). Reaction rates and polymer molecular weights (MW) are largely unaffected by low concentrations of water and carbon dioxide emphasising the stability of the propagating cyanoacrylate anion ’). Rapid polymerization is effected by ionic and covalent bases 3.4J. The sensitivity of poly(cyanoacry1ates) to degradation by basic species is well known ’-’). Most degradation studies involved correlating the rate of base-catalysed formaldehyde produc- tion, which took place over a number of days at room temperature, with proposed reaction mechanisms. In one case” low-MW (5-10 x lo3) polymer was obtained following addition of millimolar concentrations of NaOH to THF solutions of high- MW poly(BCA). The formation of the low-MW polymer was ascribed to unidenti- fied chain scission reactions.

This communication reports very rapid (seconds), tetrabutyl ammonium hydroxide (TBAOH) initiated, BCA polymerizations that are fed by monomer supplied from depolymerization of high-MW poly(BCA)’. lo). The main features of the depolymerization-repolymerization (DPRP) reaction are described, and reasons for the reaction are discussed. The behaviour of polymer formed in TBAOH initiated polymerizations of BCA monomer is also investigated. The description of the DPRP

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B. Ryan, G. McCann 218

reaction is confined to poly(BCA)/TBAOH/THF at 21 “C. However, the DPRP reaction of poly(BCA) takes place at temperatures as low as -20°C”). The reaction is initiated by amines (e. g. triethylamine), carboxylic acid salts (e. g. TBA acetate) and NaOH ‘ I ) . Ethyl, propyl, hexyl and neopentyl cyanoacrylates, and ethyl 2-cyano-2,4- pentadienoate polymers have also been shown to undergo the reaction ‘ I ) .

Experimental part High-molecular-weight (MW) parent poly(buty1 cyanoacrylate) (poly(BCA)) solution

was prepared by addition of a freshly prepared triphenylphosphine solution in tetrahydrofuran (THF, 1 x lo-’ mol/L) to a rapidly stirred 6.5 x lo-’ mom solution of the monomer in THF. The presence of weak-acid chain transfer agent (approximately lo-’ mom) in the monomer ensured that the resulting polymer was proton capped. Polymerization exotherms were obtained using an adiabatic reaction calorimeter and thermistor-measuring system of similar design to that already described”). Polymer molecular weights were determined using an acetone eluent with a combination of either three linear Poly- mer Laboratory mixed B, or two mixed B and one 500 A, gel-permeation chromatography (GPC) columns, and a Waters GPC system equipped with a 410 differential refract- ometer to monitor the eluent. All molecular weight measurements were directly based on poly(methy1 methacrylate) (PMMA) calibration standards (supplied by Polymer Labora- tories). Depolymerization experiments were performed by addition of THF solutions of tetrabutylammonium hydroxide (TBAOH), followed, after the reaction time, by addition of excess terminating agent methanesulfonic acid, from separate microsyringes to rapidly stirred THF solutions of the polymer. The microsyringes were equipped with micro- switches that facilitated accurate electronic measurement of the depolymerization time. BCA monomer was distilled at reduced pressure (b. p. 68 “C) prior to use. TBAOH and triphenylphosphine (supplied by Aldrich) were used without further purification.

Depolymerization-repolymerization (DPRP) Addition of TBAOH to a THF solution of poly(BCA) in THF at 21 “C resulted in

rapid depolymerization of the “parent” polymer accompanied by simultaneous formation of lower-molecular-weight repolymerized “daughter” polymer (Fig. 1). The number-average-molecular-weight (Mn) of the parent polymer remained almost constant during the DPRP reaction. In contrast, the Mn of the daughter polymer continuously increased with consumption of the parent polymer, although its Mn was always greater than that which would be expected from an ideal living polymerization (Fig. 2).

The ratio of weight- to number-average molecular weights MJMn of the daughter polymer was narrower than that of the parent polymer, and was considerably narrower than the most probable Flory distribution (i.e. 2) throughout the time course of the reaction (Fig. 3).

The sum of the parent and daughter polymer gel-permeation chromatography (GPC) peak areas remained almost constant during the DPRP reaction, indicating that unzipped monomer from the parent polymer was quantitatively converted to daughter polymer (Fig. 4).

Novel sub-ceiling temperature rapid depolymerization-repolymerization reactions ... 2 19

-. - .. - -

parent

t

- -

6.5 5.6 1.8 3.9 3.0 I o gto (Molecular weight 1

Fig. 1. Gel-permeation chromatography traces showing the depolymerization of parent high-molecular-weight (MW) poly(buty1 cyanoacrylate) (1.3 x m o l n referred to monomeric unit) by tetrabutylammonium hydroxide ( I .5 x lo5 m o m ) and the simultaneous formation of low-MW daughter polymer in tetrahydrofuran at 21 “C. Reaction times are indicated in the figure. The upward shift in the baseline was caused by tetrabutylammonium methanesulfonate which was formed following termination of the depolymerization-repolymerization reaction by methanesulfonic acid

,--&------.. -----------

550 500 L50

ISC 3

g 2 0

I ’ 0

Fig. 2. Least-squares plot of number-average molecular weights a,, (Fig. 1) of the parent and daughter polymers vs. conversion of the parent polymer. A hypothetical plot of daughter polymer a,, (calculated on the basis of instant quantitative initiation by tetrabutylammonium hydroxide (TBAOH) of monomer released from the parent polymer) that would have been obtained if initiation by TBAOH yielded an ideal living polymerization, is also included

220

'5 parent

Ir 1.8 - a daughter - .

B. Ryan, G. McCann

- c 2.0 -

1.6 -

1.L -

1.2 -

1.0 -1

u) 700

600 n 6 500

c .- C

C ._ 0 LOO

: 200 2 100

aJ 300

Y

a

0 0

..- -

~

7

-a-4L_----A'o/----Q

&

-.- parent -.,- daughter -A- total area

--.\.

Fig. 3. Least-squares plots of the ratio of weight- to number-average molecular weights M, fM,, of parent and daughter polymer vs. conversion of the parent polymer

Fig. 4. Gel-permeation chromatography (GPC) peak areas vs. time of the parent and daughter polymer GPC traces shown in Fig. 1

0 10 20 30 LO 50 60 Time in s

A 10-fold reduction in initial concentration [TBAOH],, increased the m,, of the daughter polymer, and decreased the rate of the DPRP reaction (Fig. 5).

Discussion

Mechanism

The near constant a,, of the parent polymer during the DPRP reaction indicates that the rate-determining step for the depolymerization process was chain-end activation. If chain unzipping was slow relative to chain-end activation, then the an of the parent polymer would decrease in direct proportion to its GPC peak area. Chain scission followed by rapid depolymerization of both scission fragments would also account for the behaviour of the parent polymer. However, it is unlikely that the latter explanation is correct, as it was shown that chloride end-capped poly(BCA) was resistant to the DPRP reaction ' I ) .

Novel sub-ceiling temperature rapid depolymerization-repolymerization reactions ... 22 1

aJ 0 C

aJ

X

Q

0, Q C

aJ

0

E c c .-

.-

.- c

2 - aJ u '

- parent polymer

/ \ I :, I \ : 'i I \ i :,

I

Fig. 5. Effect of different initial concentration of tetrabutylammonium hydroxide [TBAOH], on the depolymerization rate of the parent polymer and on the molecular weight of the daughter polymer in tetrahydrofuran at 21 "C. The parent polymer concentration was 1.3 x mol/L referred to monomeric unit. Reaction times and corre- sponding [TBAOH], are shown in the figure. The number-average molecular weights of the higher and lower molecular weight daughter polymers were 5.9 x lo3 and 2.5 x lo', respectively. Respective ratios of weight- to number-average molecular weights of daughter polymer were 1.39 and 1.2 1

A possible reaction scheme is illustrated below. Chain-end activation could occur via abstraction of the terminal proton from the parent polymer (PpH) by both the hydroxide ion, and the growing daughter polymer (Pg). Monomer (M) released by unzipping of the anionic parent polymer is consumed by the hydroxide-initiated repolymerization reaction of Pg :

k PpH+OHG @ + H , O

Ka P $ _ , + M @

k2 M + O H ~ + @

K3 @ + M = r, A b @ + M = @+,

k4 @ +PpH + P,H+@

parent polymer terminal proton abstraction

monomer/parent polymer equilibrium

initiation of daughter polymerization

monomer/oligomer equilibrium

monomer/daughter polymer equilibrium

parent polymer terminal proton abstraction by daughter polymer

222 B. Ryan, G. McCann

h

y- 2.5- n

Neglecting the very small percentage of hydroxide that is consumed in proton abstraction from the high-MW parent polymer, the reaction scheme yields the following integrated equations ‘ I ) .

/a -/

/c--

(i) If k4 = k , then no deceleration should be observed. However, a deceleration would occur if consumption of hydroxide by reinitiation was gradual, and if k4 k , . The latter inequality is not unreasonable since it would be expected that the rate constant for interchain proton transfer ( k 4 ) would be less than the rate constant ( k , ) for proton abstraction by hydroxide.

(ii) k4 could also contribute to the deceleration if it decreased with increasing molecular weight of the daughter polymer. This is not an unreasonable supposition as the rate of interchain transfer of the terminal proton between an anionic and a proton-capped polymer should be affected by the M W of the reacting polymers. Under such circumstances, a continuously decelerating first-order plot would be obtained, even if all of the hydroxide was rapidly consumed in reinitiation of daughter polymer, making the value of the integral in Eq. (2) equal to approximately [OH-],t from a very early stage in the reaction.

Consumption of hydroxide by side reactions, or a more complex reaction mechanism could also account for the deceleration. Quantitative kinetic analysis of the DPRP reaction requires absolute molecular weight measurements ‘ I ) , owing to the sensitivity of the integral in Eq. (2) to the value of [Pi]‘.

Novel sub-ceiling temperature rapid depolymerization-repolymerization reactions ... 223

Molecular weights

The measured an values of the daughter polymer were higher than those that would be expected from an ideal living polymerization (Fig. 2). However, the daughter polymerization had many of the features associated with a slowly initiated living polymerization, i. e. the increase in molecular weight both with conversion of monomer released from the parent polymer (Fig. 2), and with decreasing initiator concentration (Fig. 5), and the closeness 1 3 ) of the final a,,./an to 1.33 (Fig. 3).

Ceiling temperature and energetics

The rate of actual monomer unzipping from the parent polymer could not be measured as the rate-determining step for the unzipping process was chain-end activation. However, the rapid rate of the DPRP reaction is remarkable as the ceiling temperature for polymerization of bulk BCA monomer 1 4 ) is greater than 150°C, and following initiation by pyridine, dilute solutions of the similar, ethyl cyanoacrylate monomer, rapidly polymerize to high-MW polymer at 50°C in THFI5) and 80°C in chlorobenzene for the molar polymerization enthalpy in dilute solution ( A H , ) of BCA in IP-dioxane at 21 "C. If, as suggested by the reaction scheme, monomer unzipping is not a concerted process, then the activation energy for monomer unzipping (E-,) must be at least as large as -AH,. The additional contribution to E-, of the activation energy for the chain propagation is likely to be small as the composite propagation rate constant for BCA in THF exhibits a slight negative temperature dependence "). The implications of these observations for the DPRP reaction of poly(BCA) will be discussed in a subsequent publication 'I).

A value of -53.8 kJ/mol was determined

Explanations for the DPRP reaction

for the DPRP reaction are of fundamental interest. Although the initiation mechanism for the DPRP reaction is speculative, reasons

Stability of parent and daughter polymer

The simplest explanation for the reaction is that the parent and daughter polymers had different intrinsic stabilities owing to structural differences such as different chain tacticities, or the zwitterionic nature (owing to its initiation by triphenylphosphine) of the activated parent polymer. This explanation is not borne out by experiment because it was shown ' I ) that the daughter polymer, and parent polymer that had been polymerized by anionic initiators, were also highly susceptible to the DPRP reaction. Other explanations of a more fundamental nature are required to explain the phenomena.

224 B. Ryan, G. McCann

Flory distribution

In the absence of side reactions, a living polymer system will, with time, adopt the most probable Flory distribution via a scrambling reaction involving exchange of monomer between polymer chain ends. (It has been s h o ~ n ' ~ * ~ ~ ) that the rate at which a Poisson distribution of living polymer converts to the Flory distribution is approximately proportional to the depropagation rate constant, and inversely proportional to the Hi, of the polymer.) Although the high-MW component of the bimodal polymer distributions described in this work could not be described as living, it could be argued that the driving force for the DPRP reaction was Flory-type scrambling of the anionic high-MW polymer with anionic low-MW daughter polymer (anionic in this context means polymer containing an anionic chain end). However, at both the high and lower hydroxide concentrations (Figs. 3 and 3, the M,,,/M,, values of the respective daughter polymers were always less than the MW/Mn of the parent polymer, and were considerably less than 2 . It would, therefore, appear that explanations involving Flory-type scrambling for the DPRP reactions are invalid. (It is possible that the Hw/M,, values of the daughter polymers resulting from the experiments that were camed out at higher [TBAOH], were slightly broader than the measured values, as the low-MW component of these polymers eluted near the exclusion limit of the GPC columns. However, it is unlikely that the error in these HwlMn values was greater than 0.2, as the measured Mw/a,, of the daughter polymer that was obtained at the lower [TBAOH],, and which eluted well within the resolu- tion range of the GPC columns, was 1.39.)

Equilibrium monomer concentration

It has been ~ h o w n ~ ' - ~ ~ ) that the equilibrium monomer concentration [MI, in a dilute living polymer solution, increases with increasing number-average degree of polymerization (x,,), according to Eq. (3), where K is the monomer/polymer equilibrium constant.

[MI, = (1 - l / x , , ) / K (3)

Although the difference in the equilibrium monomer concentration for polymers with x,, values greater than 10 is very small, the equation provides an explanation for the DPRP reaction. Thus, the reaction is the relaxation process which results following perturbation of the monomer/anionic parent polymer equilibrium, by anionic low-MW daughter polymer formed from the reaction of hydroxide with monomer released from the anionic parent polymer. The monomer/polymer equilibrium is restored only when both parent and daughter polymer achieve similar molecular weights. In the absence of termination, subsequent Flory-type scrambling should, in principle, broaden the final HW/M,, of the daughter polymer. It was shown") that the DPRP reaction also occurred even at very low hydroxide concentrations, where the M,, of the daughter polymer was reduced by less than two-fold relative to the parent high an polymer. The very small predicted differences in [MI, between parent and


L a

Fig. 7. Adiabatic calori- metry curves of tetrabutylammonium hydroxide (TBAOH) initiated polymerizations of butyl cyanoacrylate ( 1.3 x 1 0-2 mol/L) in tetrahydrofuran at 2 1 "C. Initial TBAOH concentrations are indicated in the figure

daughter polymer in the latter system, raise the possibility that small variations in the monomer polymer/equilibrium constant ( K ) with x, also contribute to the DPRP reaction, the aforementioned variations only revealing themselves in reactions that are driven by difference in x,. The latter point is further discussed in the "Conclu- sions" section.

j,

- _ _ - - - /

/ /

/ /

/ /

/ __ 1.5~10.~ mol/L

1.0n10-6 mol/L /

/ /

/ /

/ 1

Polymerization/depolymerization/repolymerization (PDPRP) Apart from its ability to cause the DPRP reaction, the hydroxide ion is an effec-

tive initiator of anionic cyanoacrylate polymerizations. Fig. 7 shows the effect of different initial concentrations [TBAOH], on the polymerization rate of BCA monomer in THE The more rapid polymerization could not be accurately followed by the measuring system. The overshoot in its exotherm occurred because of slow absorp- tion of heat by the glass calorimeter walls 1 2 ) .

The polymer resulting from the slower polymerization showed no change in molecular weight for periods of at least 2 min following complete monomer consumption. In marked contrast, the high-MW polymer that initially formed in the fas- ter polymerization rapidly decreased in MW during this period (Fig. 8).

The differences in behaviour of the former and latter polymers can be explained if the initiation rate constant (ki ) is less than the composite propagation rate constant (k,,). A k i / k , value of 1.2 x lo-' for TBAOH initiated BCA polymerizations in THF at 20°C has been determined2". Despite the low k,lk, ratio, it is possible at very low TBAOH concentrations that initiation, although slow, is sufficiently rapid to allow nearly complete initiator c~nsumpt ion~~ ' . In the absence of side reactions. the M, of polymer resulting from such a polymerization would remain constant (Fig. 8). At higher TBAOH concentrations, a large proportion of the initiator would remain unreacted following nearly complete monomer conversion. Under these circumstances, the high-MW polymer that initially forms (appreciable depolymerization has already occurred within 1 s (Fig. 8)), subsequently undergoes the DPRP reaction

B. Ryan, G. McCann 226

1

01 V C

01

Q

X 01 -0 C

Q) > V

E - L ._

._

.- L

z c 01 rx

Fig. 8. Gel-permeation ‘ 1.0z10+moi/L 1.5xlO-3rnol/L chromatography traces 10s - 1.0s of polymer obtained

. . . . .. . . . . 120 s from repeat polymerizations (at both tetrabutylammonium hydroxide (TBAOH) concentrations as described in Fig. 7), which were killed by addition of excess methanesulfonic acid (3 x mom) at various times (as indicated in the figure)

__ 120s

L.2 3.3 following addition of 5.9 5.1

log,, (Molecular weight) TBAOH initiator

with the excess initiator to form lower-MW daughter polymer. The former and latter polymers could be considered as the respective kinetic and thermodynamic products of the polymerization, although strictly speaking, the daughter polymer would have to undergo further Flory-type redistribution in its molecular weight before it could be considered as the true thermodynamic product.

Conclusions Following rate-determining abstraction of the terminal chain proton by base (e. g.

hydroxide or acetate ions), alkyl (i. e. ethyl, propyl, butyl, hexyl, neopentyl) cyanoacrylate polymers in THF solution undergo a rapid (within seconds) room temperature depolymerization reaction. The unzipped monomer is simultaneously repolymerized by the base to yield lower MW “daughter” polymer that increases in MW with monomer conversion. Similarly, THF solutions of cyanoacrylate monomer undergo near-instantaneous polymerization by millimolar quantities of base yield- ing, initially, high-MW polymer that subsequently rapidly depolymerizes with simultaneous, base-initiated repolymerization of lower-MW daughter polymer. The depolymerization-repolymerization reaction was identified as the relaxation process that occurs following perturbation of the monomerhigh-MW parent polymer equilibrium by low-MW daughter polymer, and proceeds at least 100°C below the ceiling temperature of the parent polymer.

Quantitative assessment of the monomer unzipping rate was impossible owing to rate-determining abstraction of the terminal chain proton. However, there are many polymer systems where living polymer is readily prepared. Addition of further initiator or low-molecular-weight living polymer to these systems should allow direct measurements of the monomer unzipping rate and its dependence on the respective x, values of parent and daughter polymers. Such measurements may pro- vide a very sensitive method for detecting any small differences that might exist between fundamental propagatioddepropagation rate constants of otherwise identi-


cal polymers of differing chain length. Direct methods of quantifying the aforementioned differences, such as measurement of the rate of polymerization following addition of monomer to living polymer of differing x, (i.e. for x, > 5 ) involve extracting very small variations from large measured quantities. These variations are likely to be always swamped by experimental uncertainty. As the depolymerization- repolymerization reaction depends on differences in x, associated rate measurements should be immune to the aforementioned errors. Rates of daughter polymer initiation could also readily be determined in systems where the propagation centre has measurable spectral characteristics (e. g. in anionic styrene polymerizations).

Apart from the depolymerization-repolymerization reaction discussed above, the polymerization-repolymerization-depolymerization reactions which were described should also occur in slowly initiated living polymerization systems, provided that unreacted initiator remains following nearly complete monomer conversion.

’ ) D. C. Pepper, B. Ryan, Makromol. Chem. 184,395 (1983) *) D. C. Pepper, J. Polym. Sci., Polyrn. Symp. 62,65 (1978) 3, I. C. Eromosele, D. C. Pepper, Makromol. Chem., Rapid Cornmun. 7,639 (1986) 4, D. S. Johnston, D. C. Pepper, Makromol. Chem. 182,393 (1981) 5 , E. F. Donnelly, D. C. Pepper, Makromol. Chem., Rapid Commun. 2,439 (1981) h, A. W. Cooper, P. J. Harris, G. K. Kumar, J. C. Tebby, J. Polym. Sci., Purr A: Polvrn.

7, W. R. Vezin, A. T. Florence, J. Biomed. Muter: Res. 14,93 (1980) 8, E. F. Donnelly, Thesis,University of Dublin (1978) 9, B. Ryan, Internal Research Report, Loctite (Ireland Ltd.), May 1991

lo) B. Ryan, G. McCann, Contributed Lecture, I2Ih International Symposium on Ionic

‘ I ) B. Ryan, G. McCann, paper in preparation 1 2 ) J. P. Cronin, D. C. Pepper, B. Ryan, Chem. Ind. (London), 775.2 October 1982 1 3 ) V. S. Nanda, R. K. Jain, J. Polym. Sci., Part A: Gen. Pap. 2,4583 (1964) 14) J. Guthrie, Internal Research Report, Loctite (Ireland Ltd.), 1990 1 5 ) D. C. Pepper, B. Ryan, Makromol. Chem. 184,383 (1983) 1 6 ) R. Russell, Thesis, Waterford Regional Technical College (1992) 1 7 ) B. Ryan, G. McCann, Internal Research Report, Loctite (Ireland Ltd.), May 1995 I * ) J. P. Cronin, D. C. Pepper, Makromol. Chem. 189,85 (1988) 1 9 ) W. B. Brown, M. Szwarc, Trans. Furuday SOC. 34,416 (1958) 20) A. Miyake, W. H. Stockmayer, Makromol. Chem. 88,90 (1965) * I ) A. V. Tobolsky, A. Eisenberg, J. Colloid Sci. 17,49 (1962) **) J. Leonard, D. Maheux, J. Macromol. Sci. 7, 1421 (1973) 23) M. Szwarc, M. van Beylen, “Ionic Polymers and Living Polymers”, Chapman &

24) I. C. Eromosele, Thesis, University of Dublin 1987 25) M. Litt, J. Polym. Sci. 58,429 ( 1 962)

Chem. 27, 1967 (1989)

Polymerisation, September 4-8, 1995, Istanbul

Hall, New York 1993, p. 29

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